This invention relates generally to electrical signal filters, and particularly to such filters for filtering a binary information signal to be used for signal control purposes.
Video signal processing systems, such as television receivers, are increasingly using digital processing and digital control techniques. Digital control systems can be implemented in a variety of ways and using a variety of available digital processing circuitry, such as binary rate multiplier (BRM). For example, a CD 40891B BRM available from the Solid State Division of RCA Corporation, or a SN 5497 BRM available from Texas Instruments Corporation can be used for such processing.
The digital control signal for both monaural and stereo sound systems, including those for TV receivers, are typically passed through both radio frequency interference (RFI) and low pass filters before being applied to the circuit being controlled. Typically, in the prior art, the audio control signals are passed through a RFI filter having a short time constant to suppress high frequency components of the control signal. The RFI filter section is followed by an integration filter section having a longer time constant than the RFI filter. The longer time constant integrates the digital control signal to a dc value, and also prevents the control signal from causing an audible "pop" to be generated when the control signal is changed between a high level and a low level. The fully filtered control signal can be used to adjust the bass, treble and balance of the audio signal. The prior art two-section filters are disadvantageous because the integration pole having the longer time constant uses electrolytic capacitors, which are polarized, i.e. not bilateral and therefore are subject to being inserted backwards unless care is taken. Electrolytic capacitors are physically larger than unpolarized chip or disc capacitors, an important consideration in equipment using solid state devices, where size is an important consideration. Additionally, electrolytic capacitors are more expensive, which is a major consideration in the competitive field of consumer electronics, such as television receivers. For these reasons, there is a need for a combined RFI filter and a low pass filter, or dc integrator, which does not require electrolytic capacitors. The present invention fulfills these needs.